In general, an artificial leg including a knee coupling or a knee joint comprises an upper member for supporting a socket at its upper end, a lower member for supporting a foot part at its end, and a knee coupling for bendably coupling the upper and lower members, and a cylinder device for assisting or limiting the movement of the knee coupling.
As a cylinder device for assisting or limiting the movement of a knee coupling, a linear actuator is known in addition to a hydraulic cylinder, a spring cylinder and a pneumatic cylinder. The first-mentioned linear actuator generates a self-driving force by a motor. On the other hand, anyone of the various cylinders which belong to the second-mentioned group generates a resisting force against the force or movement produced by the wearer of the artificial leg. However, the form of the cylinder device is the same regardless whether the cylinder device is the first-mentioned one or anyone of the second-mentioned group. That is, the cylinder device comprises a cylinder body, which is a casing having a comparatively large diameter, containing a piston and a spring therein (in the case of the linear actuator, a body casing containing a motor therein), and a rod member, which is an elongated member having a smaller diameter than the cylinder body, extending from the cylinder body.
The conventional artificial leg having such a cylinder device as just mentioned is a sort of open-type which is provided with an inlet port and is obliged to allow foreign matters (water, sweat, dust, etc.) to enter therein from outside. The reasons why the open-type is obliged to be employed or why entrance of foreign matters is unavoidable are considered to reside in that the cylinder device, particularly the rod member thereof performs a unique movement in accordance with the movement of the artificial leg. When the upper and lower members are move or bend with respect to each other, the rod member is expanded and contracted in the axial direction of the cylinder body and at the same time, swung in the direction crossing the axis. For example, the expanding and contracting amount of the rod member is about 35 mm or larger and its swinging angle is about 10 degrees or larger.